Certain brominated diphenyl oxide mixtures are sold and denominated by the flame retardant industry as octabromodiphenyl oxide. For the sake of simplicity, these mixtures will hereinafter be referred to as "Octabrom". Typically, the commercially significant Octabroms have an average bromine number of about 7 to about 7.8 and contain 0-2 weight percent pentabromodiphenyl oxide, 5-15 weight percent hexabromodiphenyl oxide, 40-55 weight percent heptabromodiphenyl oxide, 25-40 weight percent octabromodiphenyl oxide, 5-15 weight percent nonabromodiphenyl oxide, and 0-2 weight percent decabromodiphenyl oxide. As can be appreciated, for any particular Octabrom, the average number of bromine atoms per molecule of brominated diphenyl oxide, hereinafter referred to as the "average bromine number," is dependent upon the amounts and the identities of the particular bromo species which are present in the mixture. The "average bromine number" can be calculated by multiplying the weight percent of each bromo homolog by the number of bromine atoms in that homolog, adding the resulting products and dividing the sum by 100.
One process for making Octabrom, which is described in U.S. Pat. No. 3,965,197, uses an aluminum halide to catalyze the bromination of diphenyl oxide. Another process for making Octabrom is disclosed in U.S. Pat. No. 4,740,629. While both processes represent significant advances in the art, they produce Octabrom products which are often somewhat colored. In fact, most present day Octabrom processes do not produce the desired white product, but rather, produce a product which has a noticeable reddish color component.
In general, the prior art processes for preparing Octabrom utilize a reaction mass containing diphenyl oxide, catalyst and a substantial excess of bromine. By substantial excess of bromine is meant more than 25% excess based on the stoichiometric amount of bromine needed to achieve the desired average bromine number. As the bromination of the diphenyl oxide progresses, the bromine number of the product is monitored. When the analysis of the product in the reaction mass indicates that the desired bromine number is being approached, water is added to the reaction mass to "kill" the catalyst. However, since the reaction mass contains a substantial excess of bromine, it is very difficult to "kill" the catalyst at the precise time needed to consistently prepare a product having the desired average bromine number. There is a need therefore for a process which provides a product which consistently contains the desired average bromine number.
An acceptable standard for determining the color components of an Octabrom product is provided by the use of the Hunter color values "L", "a", and "b" which can be measured with a HunterLab Spectrocolorimeter. The "L" value is a measure of lightness versus darkness or clearness versus cloudiness, higher values having greater lightness or clearness. The "a" and "b" values are measures of color. Positive "a" values indicate redness, and negative "a" values indicate greenness. Positive "b" values indicate yellowness and negative "b" values indicate blueness. Octabrom typically has HunterLab "L" values of 80 to 86, "a" values of 1 to 50 and "b" values of 18 to 22 and can have a significant reddish tint when iron is used to catalyze the reaction. Because of the wide variation in "a" values, it is difficult to maintain consistent color in thermoplastic resin formulations which incorporate Octabrom as a flame retardant.